A great number of images have come to exist on the web with the proliferation of social media having image sharing functions such as Facebook (registered trademark), Instagram (registered trademark), and Flickr (registered trademark). Further, images posted through these social media are accompanied by auxiliary information such as capture times and capture positions such that images can be searched for on the basis of various types of information. For example, Google Maps (registered trademark) implements a function of displaying images on a map at locations at which they were captured.
However, since such information is transmitted from devices of consumers, images are mostly accompanied by erroneous auxiliary information due to clocks being out of sync or the accuracy of the GPS being low or are posted with such auxiliary information being set private for protecting privacy. Thus, many methods which enables image search by learning the relationship between such information and the content of the images have been proposed. For example, Non-Patent Document 1 suggests a method in which images to which capture location information similar to that of an input image is attached are searched for and the ranks of search results are updated on the basis of capture locations of the images to estimate a capture location of the input image from capture location information attached to images ranked high in the search results.
However, the proposal of a system which similarly enables image search with respect to time information, a method which solves the lack of capture times as auxiliary information, i.e., which enables image search even for an image group including many images of time zones in which no images were captured, or the like has hardly undertaken. In some attempts in the related art, the length of time to which attention is to be paid is limited in advance to simplify complex changes of images with respect to time. Non-Patent Document 2 proposes a method of expressing representative image transitions with the lapse of time in a graph structure for each day on which common events are likely to occur all day from morning till evening as on the Independence Day of the United States. Non-Patent Document 3 proposes a method of expressing gradually changing townscape states over a long period of several years or more with a three-dimensional time-stamped model. Non-Patent Document 4 models townscape or natural transitions which are changes over medium to long terms such as several months to several years, but this can be regarded as similar to the above technologies in the sense that it discards changes over a short period such as changes over a day to simplify complex changes.